What This Calculator Does
Cable containment — trunking, tray, basket and ladder — is typically suspended from the soffit
on threaded rods and Unistrut channels. On a real riser or plant deck these supports form an
interconnected network, not independent trapezes: channels hang from rods, rods
hang from other channels, some rods are shared between two bearers, and everything accumulates
upward to a handful of ceiling fixings. This tool lets you draw that network, rolls the loads up
automatically, and checks every element against capacity.
The Six Checks
For each fire/service condition (ambient, 30, 60 or 120 minutes) the calculator checks:
rod tension against the reduced allowable stresses of BS 8519:2020 Table E.1;
top fixings (selectable ceiling anchors — Lindapter wedge nuts, flange clamps or
your own) against their condition-dependent capacities; channel bending against
manufacturer catalogue capacities (strength and deflection-limited, fire-tested columns for the
FR range) using the equivalent-UDL method; channel deflection against an L/200
serviceability limit; channel nuts against slip capacity; and a
load-conservation sanity check that catches support shares that don't sum to one.
Why Fire Cases Govern
At ambient temperature a threaded rod works at around 100 N/mm² — but in a 120-minute fire
case BS 8519 reduces that to just 6 N/mm², and a wedge-nut fixing that holds 2.1 kN
cold may be rated at only 0.4 kN. A bracket that passes comfortably at ambient can fail
badly in fire, which is why the condition switch re-checks the whole network instantly — and why
fire-rated services that must survive (alarm, evacuation, firefighting circuits) deserve their
own check with all services still hanging on the shared supports.
An Auditable Excel Record
The headline feature: Export Excel writes a workbook where every computed cell
contains a live formula — load roll-ups reference each child element's cells, so a checker can
follow the entire load path with Excel's Trace Precedents, flip the fire condition inside the
spreadsheet, and watch every PASS/FAIL recompute. Reference tables, sources and a VERIFY register
are included, because a calculation that feeds a stamped package must be checkable, not a screenshot.
Sizing the containment itself
This calculator holds the containment up; it does not size it. For the containment on the other
end of the brackets, the
cable containment fill calculator
sizes trunking, duct, tray, ladder and basket from a cable schedule and reports the cable weight
per metre you need here. The background is in
containment fill and the 45% space
factor, and the fire-support rules in
BS 8519 fire-rated containment
supports.
What does BS 8519 actually require of a support?
That the support system for a fire safety service keeps holding it up for as long as the cable itself is required to work — there is no point in a two-hour fire-rated cable hanging from a bracket that fails in twenty minutes. In practice that means fixings, rods and bearers with fire-tested performance at the required rating, and it rules out plastic plugs, plastic cable ties and unrated drop rods on those routes. The calculator checks each element at ambient and again at its fire rating.
Why is a bracket checked twice?
Because the two cases have different loads and different capacities. The ambient check uses a load factor of 1.3 against normal capacities. The fire check keeps every service still hanging — in a fire the non-fire-rated tray has not gone anywhere, so the fire-rated support still carries it — but treats fire as the accidental case with a load factor of 1.0, against fire-tested capacities. An element is only acceptable when it passes both, and the tool shows the two verdicts separately rather than merging them.
How does an element know what fire rating it needs?
It inherits the worst rating of anything it supports. Fire rating is a property of each service, not a switch on the whole job, so a bracket carrying a two-hour fire alarm cable and an ordinary small-power tray inherits the two-hour requirement for the whole load. That propagates up the network, which is why adding one fire-rated service low down can change the requirement on everything above it.
Why is deflection checked against a different load than strength?
Because they are different limit states. Strength checks — rod tension, channel bearing, the channel nut — are ultimate limit state and use the factored load. Deflection is serviceability: it asks whether the bracket sags visibly or ponds, not whether it breaks, so it is checked against the unfactored characteristic load. Ceiling anchors are also checked unfactored, because their published safe working loads already include a factor of safety and applying another would count it twice.
Can I use this for a stamped submission?
Not on its own. It is a first-pass design aid, and every reference value it uses — rod capacities, channel load tables, anchor and Lindapter figures, cable and containment weights — is indicative and carries a VERIFY note. Confirm each against the current manufacturer data and fire-test evidence for the products actually specified. The Excel export exists precisely so a checker can follow the load path and the arithmetic rather than take a screenshot on trust.
How do I work out the load per metre to put in?
It is the containment plus everything in it. The tool's containment library carries combined figures based on worst-case cable fills, which is fine for a first pass but pessimistic or optimistic depending on your job. For a real number, size the containment from the actual cable schedule in the containment fill calculator, which reports the cable weight per metre, then add the containment's own weight from its datasheet.